UV reflectors and UV-based light sources having reduced UV radiation leakage incorporating the same

ABSTRACT

UV reflectors incorporated in UV LED-based light sources reduce the amount of UV radiation emission into the surroundings and increase the efficiency of such light sources. UV reflectors are made of nanometer-sized particles having a mean particle diameter less than about one-tenth of the wavelength of the UV light emitted by the UV LED, dispersed in a molding or casting material surrounding the LED. Other UV reflectors are series of layers of materials having alternating high and low refractive indices; each layer has a physical thickness of one quarter of the wavelength divided by the refractive index of the material. Nanometer-sized textures formed on a surface of the multilayered reflector further reduce the emission of UV radiation into the surroundings. UV LED-based light sources include such a multilayered reflector disposed on an encapsulating structure of a transparent material around a UV LED, particles of a UV-excitable phosphor dispersed in the transparent material. Alternatively, the transparent material also includes nanometer-sized particles of a UV-radiation scattering material.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to ultraviolet (“UV”) reflectorsand UV-based light sources incorporating the same. More particularly,the present invention relates to visible light sources that are based onsemiconductor UV light-emitting devices and have reduced UV emission tothe surroundings.

[0002] Semiconductor light-emitting diodes and laser diodes (hereinaftercollectively called “LEDs”) have gained importance as light sources inmany applications because of their low power consumption with relativelygood luminescence efficiency and of recent advances in manufacturingtechnology of GaN-based semiconductor devices. Light sources have beendisclosed in which radiation or light from UV light- or bluelight-emitting LEDs is converted to light having generally longerwavelengths, particularly in the visible range. In the present inventiondisclosure, the terms “radiation” and “light” are used interchangeablyto mean electromagnetic radiation having wavelengths in the range fromUV to infrared; i.e, from about 100 nm to about 1 mm.

[0003] One common method for generating visible light from LEDs is todispose a phosphor composition adjacent to the LED to convert radiationemitted by the LED to visible light. LED-based lighting devices that usephosphors in a dispersed layer around the LED for light conversion oftensuffer from the undesirable halo effect and the penumbra effect. Thehalo effect occurs because of the poor mixing of light emitted by thephosphor and that by the LED. The LED generally emits light of onewavelength range; i.e., of one color, in a directional, anisotropicfashion. However, the dispersed phosphor emits light of anotherwavelength range; i.e., of a different color, isotropically (i.e., inall directions). Therefore, the light output from the system appears tohave different colors when it is viewed at different angles. When lightfrom such a light source is directed onto a flat surface, it appears asa halo of one color surrounding another color. The penumbra effect issimilar to the halo effect, except that the halo effect is an effect ofpoor color mixing, while the penumbra effect is an effect of non-uniformlight intensity. The penumbra effect causes the LED-based light sourceto appear brighter at the center than at the edges. As noted above, theLED emission is directional, while the phosphor emission is isotropic.Therefore, the overall light emitted by the LED-based light sourceappears brighter at the center because the LED chip emission intensityis greatest in this region. In order to remedy these problems, particlesof a color diffuser can be added into the phosphor layer to mix thecolors emitted by the LED and the phosphor. For example, U.S. Pat. No.6,066,861 briefly discloses the use of CaF2 as a diffuser for“optimizing the luminous pattern of the component.” Similarly, U.S. Pat.No. 6,069,440 mentions the use of a “dispersant”; such as bariumtitanate, titanium oxide, aluminum oxide, and silicon dioxide; togetherwith the phosphor for “blending colors.” However, these patents do notdisclose or suggest that dispersants are used in these or similardevices for any benefits other than color mixing or blending or what thedesired characteristics of these dispersants are.

[0004] UV light-emitting LEDs are particularly preferred in applicationsin which visible light is extracted because the color of the lightemitted by UV LED-based lamps is controlled largely by the phosphorblend since the UV LED chip does not contribute significantly to thevisible light emission. As used herein, the term “UV LEDs” means LEDsthat emit UV radiation having wavelengths less than or equal to about420 nm. However, as the wavelengths of radiation emitted by the LEDbecomes shorter, and the radiation, thus, becomes more energetic, thereis an increasing need to ensure that UV radiation preferably does notescape substantially from the lighting device into the surroundingenvironment.

[0005] U.S. Pat. Nos. 5,813,752 and 5,813,753 disclose a UV/blueLED-phosphor device that emits visible light. In U.S. Pat. No. 5,813,752the UV/blue LED is disposed on a sapphire substrate and a phosphor layeris applied directly on the UV/blue LED. A long-wave pass (“LWP”) filterpreferably composed of a multilayer dielectric stack of alternatingmaterials having high and low refractive indices is disposed directly onthe phosphor layer. In U.S. Pat. No. 5,813,753 the UV/blue LED isdisposed in a cup. In both patents, the UV/blue LED emits radiation inthe UV-to-blue wavelength range. The cup is filled with an epoxy havinga UV/blue-excitable phosphor dispersed therein that converts part of theUV/blue radiation to visible light. A LWP filter, preferably composed ofalternating layers of dielectric materials having high and lowrefractive indices, is disposed on top of the phosphor layer. The LWPfilter is believed to reflect UV/blue light back to the phosphor and totransmit visible light emitted by the phosphor. However, these patentsdo not teach the selection of the materials having high and lowrefractive indices, the design, or the construction of the multilayerLWP filter to achieve these goals. Material selection is among thecritical considerations for the success of such a filter in a device ofthis nature because the effectiveness of the filter in reflecting UVradiation depends, among other things, on the refractive index of thelayer disposed adjacent to the epoxy layer relative to the refractiveindex of the epoxy.

[0006] Therefore, there still is a need to provide improved UV-radiationreducing filters and improved UV LED-based lighting devices that allowonly a minimum amount of UV radiation leakage to the surroundingenvironment. In addition, it is also very desirable to provide a lightsource that has uniform color and light intensity and at the same timelow UV radiation leakage.

SUMMARY OF INVENTION

[0007] The present invention provides UV reflectors comprising materialsthat scatter or reflect UV radiation having wavelengths less than orequal to about 420 nm so that its transmission in the forward directionis reduced compared to the transmission of light having otherwavelengths, especially light having wavelengths in the visiblespectrum. Generally, a UV reflector of the present invention is made ofa composite material of at least two materials having differentrefractive indices. In a first embodiment, the UV reflector is acomposite structure of a first material in a particulate formsubstantially dispersed in a second solid material that is substantiallytransparent with respect to light of the visible spectrum. In a secondembodiment, the UV reflector is a layered structure of materials havingat least two different refractive indices. The refractive index of afirst material is less than or equal to and that of a second material isgreater than the refractive index of the medium through which lighttravels before impinging on the UV reflector.

[0008] The present invention also provides UV-based light sources havinga uniform color and light intensity and reduced UV radiation leakage.The UV-based light source comprises a LED emitting UV radiation, ashaped structure that comprises a molding or casting material coveringthe LED, particles of at least one phosphor that is excitable by the UVradiation emitted by the LED and particles of at least one UV-radiationscattering material dispersed substantially uniformly in at least aportion of the molding or casting material. The particles of thephosphor composition and the scattering material are disposed in thevicinity of the LED. The molding or casting material may be a glass or apolymeric material that is substantially transparent after curing. Asused herein, a substantially transparent material is defined as one thattransmits greater than 80% of incident light having a wavelength of 555nm at an incident angle of less than 10 degrees.

[0009] In another aspect of the present invention, the UV-radiationscattering material has a refractive index substantially different fromthat of the molding or casting material. The refractive index or indexof refraction of a material, as referred to herein, is that measured forlight having the wavelength of 555 nm.

[0010] In another aspect of the present invention, the UV-radiationscattering material is a dielectric material of which greater than 95%of a representative population of particles has particle diameters lessthan about half of the maximum wavelength of UV radiation in the moldingor casting material and which has a mean particle diameter less thanabout one-tenth of the same wavelength. The diameter of a particlehaving irregular shape is defined herein as the diameter of a sphereequaling the largest dimension of the particle. The mean particlediameter is the average particle diameter of a representative sample ofthe particles.

[0011] In another aspect of the invention, the UV-based light sourcefurther comprises a UV reflector that comprises a plurality of layers ofmaterials having at least alternating first and second refractiveindices and that is disposed on the shaped structure of a molding orcasting material. The first refractive index is greater than about 1.5,and the second refractive index is less than about 2. Each of the layershas a thickness of one-quarter or an even multiple of one-quarter of thewavelength of the radiation to be transmitted.

[0012] In still another aspect of the present invention, the surface ofthe UV reflector adjacent to the molding or casting material has aplurality of protrusions that have a typical dimension of much less thanthe wavelength of UV radiation in the molding material.

[0013] Other features and advantages of the present invention will beapparent from a perusal of the following detailed description of theinvention and the accompanying drawings in which the same numerals referto like elements.

BRIEF DESCRIPTION OF DRAWINGS

[0014]FIG. 1 schematically shows the first embodiment of the presentinvention wherein the UV reflector comprises a plurality of UV-radiationscattering material dispersed among particles of a phosphor.

[0015]FIG. 2 schematically shows the second embodiment of the presentinvention wherein the UV reflector is a layer of a UV-radiationscattering material applied over a layer of phosphor.

[0016]FIG. 3 shows the relative reflectance of 4 nm sized colloidalsilica with respect to wavelength.

[0017]FIG. 4 schematically shows a UV LED-based light source of thepresent invention with a UV reflector comprising a layer of UV-radiationscattering material disposed on another layer that comprises bothphosphor and scattering material.

[0018]FIG. 5 schematically shows a UV LED-based light source of thepresent invention including a multilayer UV reflector.

[0019]FIG. 6 shows details of the multilayer UV reflector of FIG. 5.

[0020]FIG. 7 schematically shows a multilayer UV reflector havingprotrusions used in a UV LED-based light source of the presentinvention.

DETAILED DESCRIPTION

[0021] Visible light sources based on UV LEDs typically comprise aphosphor composition disposed adjacent to a LED to convert the UVradiation to visible light. The phosphors used in the present inventionabsorb UV radiation energy and reemit light having longer wavelengths,typically in the visible spectrum. The combination of the UV LED and thephosphor is typically enclosed in a substantially transparent shapedstructure of a molding or casting material, such as glass, epoxy,silicone, or urea formaldehyde thermoset resin. Alternatively, thephosphor particles are often dispersed in the molding or castingmaterial to provide a better dispersion of light. Typically, the amountof phosphor particles used is small. Therefore, there is a considerableprobability for UV light to escape from the light source without beingabsorbed and converted by the phosphor particles. This unconverted UVradiation not only lowers the light output of the light source but alsopresents a safety concern. Moreover, UV radiation often has adverseeffect on the long-term integrity of polymeric molding or castingmaterials because UV radiation often degrades polymeric materials.Therefore, it is very desirable to reduce the amount of unconverted UVradiation traversing the polymeric material. The present inventionprovides light sources based on UV LEDs that have increased utilizationof UV radiation and reduced UV radiation leakage into the surroundingsby increasing the opportunity for UV radiation to scatter within thepolymeric material and/or to reflect at the viewing boundary of thelight source.

[0022]FIG. 1 shows the first embodiment of the present invention, alight source 10 based on a UV LED comprises a UV LED 20 located on areflective surface 24, particles 26 of a phosphor composition disposedin the vicinity of the UV LED 20 to convert UV radiation emitted by theLED 20 into visible light, particles 28 of a scattering materialdisposed among the particles 26 of the phosphor to scatter any UVradiation that is not absorbed initially by the phosphor so to give itfurther opportunity to be absorbed by the phosphor particles 26. The UVLED 20 emits UV radiation having wavelengths in the range from about 100nm to about 420 nm, preferably from about 200 nm to about 420 nm, morepreferably from about 300 nm to about 420 nm. The particles 26 of thephosphor composition and the particles 28 of the scattering material maybe dispersed together in a molding or casting material 22 and anencapsulating structure 30 of the composite of molding material,phosphor, and scattering material is formed over the LED to provide aprotective structure thereto. In addition, a lens 32 made of atransparent material may be formed around the structure 30 to providefurther protection. The material of the lens 32 may be the same as themolding or casting material of the encapsulating structure 30. In FIG.1, the reflective surface 24 is shown as a flat surface, but the UV LED20 may also be located in a cup having a reflective surface.Alternatively, FIG. 2 shows a second embodiment of the presentinvention, a layer 40 of phosphor material may be applied to the surfaceof the UV LED 20 from which UV radiation is to emit. A layer 42 ofscattering material is applied on top of the phosphor layer 40. Each ofthe phosphor and scattering layers 40 and 42 may comprise a mixture ofthe molding or casting material and the phosphor or scattering material.The phosphor layer 40 may also contain particles of a scatteringmaterial. Then, the LED 20 with the phosphor and scattering layers 40and 42 is sealed inside an encapsulating structure 50 of the molding orcasting material. Alternatively, the encapsulating structure 50 may bemolded or cast separately and then attached to the LED 20 on which thephosphor and scattering layers 40 and 42 have been applied. In eitherconfiguration, any UV radiation that is not absorbed by the phosphormaterial is scattered by the scattering particles and is reflected backto the phosphor material for further absorption and conversion tovisible light. Thus, the amount of UV radiation leakage to thesurroundings is significantly reduced. In addition, because a largerfraction of the UV radiation is eventually converted to visible light,the light output of the light source is higher than otherwise possible,and since the UV light undergoes multiple scattering events, the visiblelight output is more uniformly distributed.

[0023]FIG. 3 shows the relative reflectance of 4 nm colloidal silicaparticles (refractive index of about 1.4) in water (refractive index ofabout 1.3). Particles having a characteristic dimension less than aboutone-tenth of the light wavelength fall within the region in which theRaleigh scattering theory applies. In this region, for non-absorbingparticles, scattering is the dominant mechanism.

[0024] The intensity of scattered light in this case varies as λ⁻⁴,where λ is the wavelength of light. See; for example, Richard Tilley,“Colour And the Optical Properties of Materials,” John Wiley & Sons Ltd,pp. 110-113 (2000). In other words, when the scattering material isnon-absorbing and the particles have a characteristic dimension, such asdiameter, less than one-tenth of an average wavelength, light of shorterwavelengths is scattered more efficiently than light of longerwavelengths. Therefore, it is preferable to use scattering particles ofdielectric materials having a mean particle diameter of less than about40 nm to scatter UV radiation.

[0025] It is well known that for Raleigh scattering, the intensity ofscattered light is proportional to [(m⁻¹)/(m²+2)]², where m is the ratioof the refractive index of the particle n_(p) and that of the mediumn_(m), or n_(p)/n_(m). See; for example, Richard Tilley, “Colour And theOptical Properties of Materials,” John Wiley & Sons Ltd, pp. 110-113(2000). Therefore, the intensity of scattered light increases as theratio m increasingly deviates from 1. Most materials suitable for use asmolding or casting materials have refractive indices of about 1.5. Manycrystalline solids, including phosphors, have refractive indicessubstantially higher than 1.5 and are suitable as scattering materials.A few solid halides have refractive indices less than 1.5. Examples ofsuitable scattering materials for the present invention along with theirrefractive indices are shown in Table 1 below. Values for refractiveindices of some materials may be found in, for example, Ronald W.Waynant and Marwood N. Ediger (Ed.), “Electro-Optics Handbook,”McGraw-Hill, Inc., pp. 1.13-11.38 (2000); Eugene Hecht, “Optics,”Addison Wesley Longman, pp. 72-73 and 94 (1998); Warren J. Smith,“Modern Optical Engineering,” McGraw-Hill, Inc., pp. 189-201. Othermaterials that have refractive indices greater than about 1.7 or lessthan about 1.4 are also preferred. TABLE 1 Approximate Refractive IndexMaterial in the UV-Visible Range Lead selbutide 5.7 Lead selmolar 4.2Lead sulfide 4.1 Germanhml 4.0 Gallium phosphide 3.5 Silicon 3.4 Iodiumanalide 3.4 Gallium anamide 3.3 Aluminum arsentide 2.9 Rtitle (TiO₂) 2.9Boron phosphide 2.8 Cadmium sulfide 2.7 Calcium tolluride 2.7 Zinctellhide 2.3 Thallium bromoiodide 2.6 Calcite (CaCO₂) 2.6 Siliconcarbide 2.6 Cadmium haptesium sulfide 2.6 Stroplurea dispate (SrTiO₂)2.5 Thallium chlorophosphate 2.4 Diamcien 2.4 Fabillite (SrTiO₂) 2.4Zinc zelenide 2.4 Cadmium selenide 2.4 Gallium nitride 2.4 Aluminumnitride 2.2 Lithleum niobate 2.2 Potassium niobate 2.2 Tatfalumpentotide 2.2 Geramium oxide 2.2 Silver chloride 2.2 Halaium oxide 2.1Liblumiolate 1.9 Zircon (ZrO₂SiOH₂) 1.9 Yitrium oxide 1.9 Siliconmonoxide 1.9 Sapphire (single crystal Al₂O₃) 1.8 Celium oxide 1.8Lanthroium fibot glass 1.8 Cesium bromide 1.7 Sodium iodide 1.7Magnesium oxide 1.7 Desine first gists 1.7 Lanhanum rilloxide 1.6 Silica(SiO₂) 1.4-1.5 Lithium fluoride 1.4 Magnesium fluoride 1.4 Potassiumfluoride 1.4 Sodium fluoride 1.3

[0026] In another embodiment of the present invention as shown in FIG.4, the light source of FIG. 1 further includes a layer 60 of at leastone scattering material disposed on the encapsulating structure 30. Thescattering layer 60 may comprise the same or different scatteringmaterial than that dispersed in the encapsulating structure 30.Furthermore the scattering material of layer 60 may be dispersed in thesame material as or different material than the molding or castingmaterial of the encapsulating structure 30, as long as it does notsubstantially affect the visible light transmission. The scatteringlayer 60 further scatters any unabsorbed UV radiation that may escapeconversion to visible light in the encapsulating structure back to thephosphor material, resulting in further reduction of UV radiationleakage.

[0027] In another embodiment of the present invention, the scatteringlayer is a layer of a molding or casting material in which air bubblesof nanometer sizes are formed. The low refractive index of air (about 1)relative to that of the molding or casting material promotes aneffective reflection of light at the interface between the air bubblesand the surrounding molding or casting material.

[0028] In still another embodiment of the present invention, the UVreflector is a multilayer distributed Bragg reflector (“DBR”) 70 that isdisposed opposite the LED and away from the phosphor layer in theviewing direction. For example, FIG. 5 shows a DBR 70 disposed on theencapsulating structure 30, which comprises particles 26 of a phosphorand particles 28 of at least a scattering material dispersed in amolding or casting material 22. The DBR 70 is a stack or a series of anodd number of alternating layers 72 and 74 of materials having high andlow refractive indices. FIG. 6 shows the DBR 70 in more details.High-refractive index materials are those having refractive indicesgreater than about 1.5, preferably greater than about 1.7, morepreferably greater than about 2, and most preferably greater than about2.3. Low-refractive index materials are those having refractive indicesless than about 2, preferably less than about 1.5, more preferably lessthan about 1.4, and most preferably less than about 1.3. Materialsenumerated above in Table 1 are suitable for the manufacturing of DBRs.Dielectric and non-absorbing materials are preferred. It is well knownthat the combined reflection of light from two surfaces of a thin filmis minimum when the optical thickness of the thin film is equal toone-quarter or an even multiple of one-quarter of the wavelength of theincident light. The optical thickness is the product of the physicalthickness t of the film and the refractive index n of the material ofthe film measured at the wavelength of the transmitted light. In thesimplest case, the optical thickness is one quarter of the wavelength ofthe incident light. It is commonly referred to as a quarter-wave layer.Each of the layers of the DBR 70 preferably has one quarter-wave opticalthickness for visible light, for example one-quarter of the lightwavelength of 555 nm. Such a film allows a minimum reflection of lightmost sensitive to the human eyes but more reflection of light havingother wavelengths, especially UV light. Therefore, a DBR having multiplequarter-wave layers of appropriate alternating materials will allow moreUV radiation to reflect at interfaces between pairs of layers. Thenumber of layers of the DBR 70 should be optimized for a minimum UVradiation leakage and for a maximum visible light transmission.Typically, a DBR comprising at least five layers would significantlyreduce the amount of UV leakage. However, depending on thecircumstances, a DBR may have fewer than five layers. A DBR preferablycomprises an odd number of layers. The DBR more preferably comprises anodd number of layers greater than or equal to 11. However, in somecircumstances, an even number of layers may also be used. For example,if n_(H) and n_(L) denote the high and low refractive index,respectively, the physical thicknesses of the layers of the high and lowrefractive index are preferably:t_(H)=(540 nm)/(4 n_(H)) to (580 nm)/(4n_(H))and t_(L)=(540 nm)/(4 n_(L)) to (580 nm)/(4 n_(L)) Furthermore, itis well known that as light travels from a first medium having arefractive index n1 to an adjacent second medium having a refractiveindex n2, a total reflection occurs when the angle of the incident lightmade with the normal to the interface at the point where the incidentlight strikes the interface exceeds a critical angle θc that satisfiesthe equation:sin θ_(c)=(n₂/n₁) As n₂ decreases, the critical angledecreases, and, therefore, more light is reflected at the interfacebetween the media. Therefore, if the first quarter-wave layer adjacentto the surface of the encapsulating structure has a refractive indexlower than that of the molding or casting material, more UV radiationwill be reflected back toward the layer containing the phosphorparticles. This layer preferably is made of a material having the lowestrefractive index among the chosen materials for the DBR, such as sodiumfluoride. The other layers having low refractive indices may comprisethe same material or different material than that of the first layer.Moreover, more than one high- and low-refractive index materials may bechosen.

[0029] In another embodiment of the present invention as shown in FIG.7, the surface 100 of the DBR 70 adjacent to the encapsulating structure30 has a plurality of nanometer-sized protrusions 110 directed towardthe LED 20. The protrusions 110 further increase the reflection at theinterface 104 between the DBR 70 and the encapsulating structure 30because the incident angle at a particular protrusion 110 may be largerthan the critical angle, while it may have been smaller than thecritical angle without the protrusion. The protrusions 110 preferablyhave a characteristic dimension such as a height or a width that isequal to about a quarter of the wavelength of 555 nm. The protrusionspreferably have conical, pyramidal, or hemispherical shape having aheight and the largest cross-sectional dimension of the baseapproximately equal to one quarter of the wavelength.

[0030] In one aspect of the present invention, the multilayer DBR isformed by depositing alternating layers of low and high refractiveindices on the encapsulating structure surrounding the LED. A firstlayer of a material having a low refractive index n_(L) in the rangefrom about 1.05 to about 1.4, such as sodium fluoride, is deposited onthe encapsulating structure by physical vapor deposition, chemical vapordeposition, or sputtering to a physical thickness t_(L)=(λ/4 n_(L))where λ is wavelength of light to be transmitted. λ may be chosen to bethe wavelength of light most sensitive to the human eyes; i.e., 555 nm.Next, a layer of a material having a high refractive index n_(H) isdeposited on the layer having a low refractive index to a physicalthickness of t_(H)=(λ/4 n_(H)) by physical vapor deposition, chemicalvapor deposition, or sputtering. Then, alternating layers having low andhigh refractive indices are formed successively on a previously formedlayer until the desired number of layers is achieved.

[0031] In another aspect of the present invention, indentations havingconical, pyramidal, or hemispherical shape and having characteristicdimensions of one quarter of the wavelength of yellow light are formedinto the surface of the encapsulating structure. A first material havinga low refractive index n_(L) in the range from about 1.05 to about 1.4is deposited on the encapsulating structure by physical deposition,chemical deposition, or sputtering to fill the indentations and also tofurther form a first layer having a physical thickness t_(L)=(λ/4 n_(L))where λ is the wavelength of the light to be transmitted. λ may bechosen to be the wavelength of light most sensitive to the human eyes;i.e., 555 nm. Then alternating layers having high and low refractiveindices are formed successively over this layer as described above. Itmay be advantageous to cover the finished DBR with a thin layer of themolding or casting material to protect the DBR.

[0032] Alternatively, the DBR may be formed separately and thensubsequently attached to the encapsulating structure either after thecomplete encapsulating structure has been formed or while it is beingformed over the LED. For example, a portion of the encapsulatingstructure may be formed first. Then the DBR is fixed on that unfinishedencapsulating portion. Finally, the encapsulating structure iscompleted. In this way, the DBR is embedded within the encapsulatingstructure.

[0033] While various embodiments are described herein, it will beappreciated from the specification that various combinations ofelements, variations, equivalents, or improvements therein may be madeby those skilled in the art, and are still within the scope of theinvention as defined in the appended claims.

1. A UV reflector comprising particles of a UV-radiation scatteringmaterial dispersed in a substantially transparent material, saidUV-radiation scattering material having a different refractive indexthan the refractive index of said substantially transparent material,said refractive indices being measured for light having wavelength of555 nm.
 2. The UV reflector according to claim 1, wherein saidscattering material is selected from the group consisting of leadtelluride, lead selenide, lead sulfide, germanium, gallium phosphide,silicon, indium arsenide, gallium arsenide, aluminum arsenide, rutile,boron phosphide, cadmium sulfide, cadmium telluride, zinc telluride,thallium bromoiodide, calcite, silicon carbide, cadmium lanthanumsulfide, strontium titanate, thallium chlorobromide, diamond, fabulite,zinc selenide, cadmium selenide, gallium nitride, aluminum nitride,lithium niobate, potassium niobate, germanium oxide, silver chloride,hafnium oxide, lithium iodate, zircon, yttrium oxide, silicon monoxide,sapphire, cesium iodide, lanthanum flint glass, cesium bromide, sodiumiodide, magnesium iodide, dense flint glass, lanthanum trifluoride,silica, lithium fluoride, magnesium fluoride, potassium fluoride, sodiumfluoride, and mixtures thereof.
 3. The UV reflector according to claim1, wherein said refractive index of said UV-radiation scatteringmaterial is less than the refractive index of said substantiallytransparent material.
 4. The reflector according to claim 3, whereinsaid UV-radiation scattering material is selected from the groupconsisting of lithium fluoride, magnesium fluoride, potassium fluoride,sodium fluoride, and mixtures thereof.
 5. The UV reflector according toclaim 1, wherein said UV-radiation scattering material is a particulatedielectric material of which greater than 95% of the particle populationhas particle diameter less than about half of a maximum wavelength of UVradiation in said substantially transparent material and which has amean particle diameter less than about one-tenth of said wavelength. 6.A UV reflector comprising a plurality of layers of materials having atleast alternating first and second refractive indices, wherein saidfirst refractive index is smaller than said second refractive index. 7.The UV reflector according to claim 6, wherein said first refractiveindex is less than the refractive index of a medium through which anincident light travels before impinging on said UV reflector, and saidsecond refractive index is greater than the refractive index of saidmedium.
 8. The UV reflector according to claim 6, wherein said firstrefractive index is less than about 2, preferably less than 1.5, morepreferably less than about 1.4, and most preferably less than about 1.3.9. The UV reflector according to claim 8, wherein said material havingsaid first refractive index is selected from the group consisting oflithium iodate, zircon, sapphire, cesium iodide, lanthanum flint glass,sodium iodide, magnesium iodide, dense flint glass, silica, lithiumfluoride, magnesium fluoride, potassium fluoride, sodium fluoride, andmixtures thereof.
 10. The UV reflector according to claim 6, whereinsaid second refractive index is greater than about 1.5, preferablygreater than about 1.7, more preferably greater than about 2, and mostpreferably greater than 2.3.
 11. The UV reflector according claim 10,wherein said material having said second refractive index is selectedfrom the group consisting of lead telluride, lead selenide, leadsulfide, germanium, gallium phosphide, silicon, indium arsenide, galliumarsenide, aluminum arsenide, rutile, boron phosphide, cadmium sulfide,cadmium telluride, zinc telluride, thallium bromoiodide, calcite,silicon carbide, cadmium lanthanum sulfide, strontium titanate, thalliumchlorobromide, diamond, fabulite, zinc selenide, cadmium selenide,gallium nitride, aluminum nitride, lithium niobate, potassium niobate,germanium oxide, silver chloride, hafnium oxide, lithium iodate, zircon,yttrium oxide, silicon monoxide, sapphire, cesium iodide, lanthanumflint glass, cesium bromide, sodium iodide, magnesium iodide, denseflint glass, and mixture thereof.
 12. The UV reflector according toclaim 6, wherein a first layer of said plurality of layers comprises amaterial having said first refractive index, said first layer receivingan incident light having wavelengths in a UV-to-visible spectrum. 13.The UV reflector according to claim 12 further comprising an additionallayer of a material having a third refractive index less than said firstrefractive index, said additional layer being attached to said firstlayer to receive said incident light.
 14. The UV reflector according toclaim 12, wherein each of said plurality of layers has a physicalthickness in a range from about 540 k/(4 n) to about 580 k/(4 n),wherein k is a positive integer number selected from the groupconsisting of 1 and positive even integer numbers and wherein n is arefractive index of a material of said layer measured at lightwavelength of 555 nm.
 15. The UV reflector according to claim 14,wherein said plurality of layers comprises an odd number of layers. 16.The UV reflector according to claim 15, wherein said odd number oflayers is greater than or equal to
 5. 17. The UV reflector according toclaim 15, wherein said odd number of layers is preferably greater thanor equal to
 11. 18. The UV reflector according to claim 12 furthercomprising a plurality of protrusions formed on a surface of said UVreflector on which an incident light impinges.
 19. The UV reflectoraccording to claim 18, wherein said protrusions are made of the samematerial as said first layer.
 20. The UV reflector according to claim19, wherein said protrusions have a shape selected from the groupconsisting of conical, pyramidal, and hemispherical shapes.
 21. The UVreflector according to claim 18, wherein said protrusions have a largestdimension in a range from about 540 k/(4 n) to about 580 k/(4 n),wherein k is a positive integer number selected from the groupconsisting of 1 and positive even integer numbers and wherein n is arefractive index of a material of said protrusions measured at a lightwavelength of 555 nm.
 22. The UV reflector according to claim 21,wherein said largest dimension is a height of said protrusions.
 23. TheUV reflector according to claim 21, wherein said largest dimension is alargest cross-sectional dimension of a base of said protrusions.
 24. TheUV reflector according to claim 13 further comprising a plurality ofprotrusions formed on a surface of said additional layer on which saidincident light impinges.
 25. The UV reflector according to claim 24,wherein said protrusions are made of the same material as saidadditional layer.
 26. The UV reflector according to claim 25, whereinsaid protrusions have a shape selected from the group consisting ofconical, pyramidal, and hemispherical shapes.
 27. The UV reflectoraccording to claim 24, wherein said protrusions have a largest dimensionin a range from about 540 k/(4 n) to about 580 k/(4 n), wherein k is apositive integer selected from the group consisting of 1 and positiveeven integer numbers and wherein n is a refractive index of a materialof said protrusion measured at a light wavelength of 555 nm.
 28. The UVreflector according to claim 27, wherein said largest dimension is aheight of said protrusions.
 29. The UV reflector according to claim 27,wherein said largest dimension is a largest cross-sectional dimension ofa base of said protrusions.
 30. A light source based on at least one UVLED, said light source comprising: a LED emitting UV radiation; a shapedstructure that comprises a substantially transparent molding or castingmaterial, said shaped structure covering said LED; particles of at leastone phosphor that is excitable by said UV radiation, said phosphordisposed to receive said UV radiation and to convert said UV radiationto visible light; and particles of at least one UV-radiation scatteringmaterial disposed among said particles of said phosphor; wherein saidparticles of said phosphor and said particles of said scatteringmaterial are dispersed in at least a portion of said molding or castingmaterial and wherein said particles are disposed in a vicinity of saidLED.
 31. The light source based on at least one UV LED according toclaim 30, wherein said UV-radiation scattering material has a refractiveindex different from the refractive index of said molding or castingmaterial, said refractive indices being measured for light havingwavelength of 570 nm.
 32. The light source based on at least one UV LEDaccording to claim 31, wherein said scattering material is selected fromthe group consisting of lead telluride, lead selenide, lead sulfide,germanium, gallium phosphide, silicon, indium arsenide, galliumarsenide, aluminum arsenide, rutile, boron phosphide, cadmium sulfide,cadmium telluride, zinc telluride, thallium bromoiodide, calcite,silicon carbide, cadmium lanthanum sulfide, strontium titanate, thalliumchlorobromide, diamond, fabulite, zinc selenide, cadmium selenide,gallium nitride, aluminum nitride, lithium niobate, potassium niobate,germanium oxide, silver chloride, hafnium oxide, lithium iodate, zircon,yttrium oxide, silicon monoxide, sapphire, cesium iodide, lanthanumflint glass, cesium bromide, sodium iodide, magnesium iodide, denseflint glass, lanthanum trifluoride, silica, lithium fluoride, magnesiumfluoride, potassium fluoride, sodium fluoride, and mixtures thereof. 33.The light source based on at least one UV LED according to claim 31,wherein said refractive index of said UV-radiation scattering materialis less than the refractive index of said substantially transparentmolding or casting material.
 34. The light source based on at least oneUV LED according to claim 33, wherein said UV-radiation scatteringmaterial is selected from the group consisting of lithium fluoride,magnesium fluoride, potassium fluoride, sodium fluoride, and mixturesthereof.
 35. The light source based on at least one UV LED according toclaim 31, wherein said UV-radiation scattering material is a particulatedielectric material of which greater than 95% of the particle populationhas a particle diameter less than about half of a maximum wavelength ofUV radiation in said substantially transparent molding or castingmaterial and which has a mean particle diameter less than aboutone-tenth of said wavelength.
 36. The light source based on at least oneUV LED according to claim 30 further comprising a additional layer thatcomprises a particulate UV-radiation scattering material dispersed in asubstantially transparent molding or casting material, said additionallayer being disposed on said shaped structure in a direction of aviewer.
 37. A light source based on at least one UV LED, said lightsource comprising: a LED emitting UV radiation; a shaped structure thatcomprises a substantially transparent molding or casting material, saidshaped structure covering said LED; particles of at least one phosphorthat is excitable by said UV radiation, said phosphor disposed toreceive said UV radiation and to convert said UV radiation to visiblelight; and a plurality of air bubbles having nanometer sizes, said airbubbles being formed among said particles of said phosphor in saidsubstantially transparent molding or casting material; wherein saidparticles of said phosphor and said air bubbles are dispersed in atleast a portion of said molding or casting material and are disposed ina vicinity of said LED.
 38. A light source based on at least one UV LED,said light source comprising: a LED emitting UV radiation; a shapedstructure that comprises a substantially transparent molding or castingmaterial, said shaped structure covering said LED; particles of at leastone phosphor that is excitable by said UV radiation, said phosphordisposed in a vicinity of said LED to receive said UV radiation and toconvert said UV radiation to visible light, said particles of saidphosphor dispersed in a portion of said molding or casting material; anda UV reflector comprising a plurality of layers of materials having atleast alternating first and second refractive indices, said firstrefractive index being smaller than said second refractive index, saidUV reflector being disposed on said shaped structure in a direction of aviewer, wherein a first layer of said plurality of layers is disposedadjacent to said shaped structure.
 39. The light source based on atleast one UV LED according to claim 38, wherein said first refractiveindex is smaller than the refractive index of said molding or castingmaterial, and said second refractive index is greater than therefractive index of said molding or casting material.
 40. The lightsource based on at least one UV LED according to claim 38, wherein saidfirst refractive index is preferably less than about 1.5, morepreferably less than about 1.4, and most preferably less than about 1.3;and said second refractive index is preferably greater than about 1.7,more preferably greater than about 2, and most preferably greater thanabout 2.3.
 41. The light source based on at least one UV LED accordingto claim 38, wherein said material having said first refractive index isselected from the group consisting of lithium iodate, zircon, sapphire,cesium iodide, lanthanum flint glass, sodium iodide, magnesium iodide,dense flint glass, silica, lithium fluoride, magnesium fluoride,potassium fluoride, sodium fluoride, and mixtures thereof.
 42. The lightsource based on at least one UV LED according to claim 38, wherein saidmaterial having said second refractive index is selected from the groupconsisting of lead telluride, lead selenide, lead sulfide, germanium,gallium phosphide, silicon, indium arsenide, gallium arsenide, aluminumarsenide, rutile, boron phosphide, cadmium sulfide, cadmium telluride,zinc telluride, thallium bromoiodide, calcite, silicon carbide, cadmiumlanthanum sulfide, strontium titanate, thallium chlorobromide, diamond,fabulite, zinc selenide, cadmium selenide, gallium nitride, aluminumnitride, lithium niobate, potassium niobate, germanium oxide, silverchloride, hafnium oxide, lithium iodate, zircon, yttrium oxide, siliconmonoxide, sapphire, cesium iodide, lanthanum flint glass, cesiumbromide, sodium iodide, magnesium iodide, dense flint glass, and mixturethereof.
 43. The light source based on at least one UV LED according toclaim 38, wherein said first layer of said plurality of layers comprisesa material having said first refractive index, said first layerreceiving an incident light traversing said shaped structure.
 44. Thelight source based on at least one UV LED according to claim 38, whereinsaid UV reflector further comprises an additional layer of a materialhaving a third refractive index less than said first refractive index,said additional layer being attached to said first layer to receive saidincident light.
 45. The light source based on at least one UV LEDaccording to claim 43, wherein each of said plurality of layers has aphysical thickness in a range from about 540 k/(4 n) to about 580 k/(4n), wherein k is a positive integer selected from the group consistingof 1 and positive even integer numbers and wherein n is a refractiveindex of a material of said layer measured at light wavelength of 555nm.
 46. The light source based on at least one UV LED according to claim43, wherein said plurality of layers comprises an odd number of layers.47. The light source based on at least one UV LED according to claim 43,wherein said odd number of layers is greater than or equal to
 5. 48. Thelight source based on at least one UV LED according to claim 43, whereinsaid odd number of layers is preferably greater than or equal to
 11. 49.The light source based on at least one UV LED according to claim 43,wherein said UV reflector further comprises a plurality of protrusionsformed on a surface of said UV reflector on which an incident lighttraversing said shaped structure impinges.
 50. The light source based onat least one UV LED according to claim 43, wherein said protrusions aremade of the same material as said first layer.
 51. The light sourcebased on at least one UV LED according to claim 50, wherein saidprotrusions have a shape selected from the group consisting of conical,pyramidal, and hemispherical shapes.
 52. The light source based on atleast one UV LED according to claim 49, wherein said protrusions have alargest dimension in a range from about 540 k/(4 n) to about 580 k/(4n), wherein k is a positive integer number selected from the groupconsisting of 1 and positive even integer numbers and wherein n is arefractive index of a material of said protrusions measured at a lightwavelength of 555 nm.
 53. The light source based on at least one UV LEDaccording to claim 52, wherein said largest dimension is a height ofsaid protrusions.
 54. The light source based on at least one UV LEDaccording to claim 52, wherein said largest dimension is a largestcross-sectional dimension of a base of said protrusions.
 55. The lightsource based on at least one UV LED according to claim 44 furthercomprising a plurality of protrusions formed on a surface of saidadditional layer on which said incident light impinges.
 56. The lightsource based on at least one UV LED according to claim 55, wherein saidprotrusions are made of the same material as said additional layer. 57.The light source based on at least one UV LED according to claim 56,wherein said protrusions have a shape selected from the group consistingof conical, pyramidal, and hemispherical shapes.
 58. The light sourcebased on at least one UV LED according to claim 55, wherein saidprotrusions have a largest dimension in a range from about 540 k/(4 n)to about 580 k/(4 n), wherein k is a positive integer selected from thegroup consisting of 1 and positive even integer numbers and wherein n isa refractive index of a material of said protrusion measured at a lightwavelength of 555 nm.
 59. The light source based on at least one UV LEDaccording to claim 58, wherein said largest dimension is a height ofsaid protrusions.
 60. The light source based on at least one UV LEDaccording to claim 58, wherein said largest dimension is a largestcross-sectional dimension of a base of said protrusions.
 61. The lightsource based on at least one UV LED according to claim 38 furthercomprising particles of a UV-radiation scattering material dispersedamong said particles of said phosphor, said particles of saidUV-radiation scattering material and said phosphor being dispersed in aportion of said molding or casting material.
 62. A light source based onat least one UV LED, said light source comprising: a LED emitting UVradiation; a shaped structure that comprises a substantially transparentmolding or casting material, said shaped structure covering said LED;particles of at least one phosphor that is excitable by said UVradiation, said phosphor disposed in a vicinity of said LED to receivesaid UV radiation and to convert said UV radiation to visible light;particles of at least one UV-radiation scattering material disposedamong said particles of said phosphor, said particles of said phosphorand said particles of said UV-radiation scattering material beingdispersed in a portion of said molding or casting material; and a UVreflector comprising: a plurality of layers of materials having at leastalternating first and second refractive indices, said first refractiveindex being smaller than said second refractive index; and a pluralityof protrusions formed on a surface of said UV reflector adjacent to saidshaped structure; said UV reflector being disposed on said shapedstructure in a direction of a viewer.
 63. The light source based on atleast one UV LED according to claim 62, wherein: said UV-radiationscattering material has a different refractive index than the refractiveindex of said molding or casting material; said UV-radiation scatteringmaterial is selected from the group consisting of lead telluride, leadselenide, lead sulfide, germanium, gallium phosphide, silicon, indiumarsenide, gallium arsenide, aluminum arsenide, rutile, boron phosphide,cadmium sulfide, cadmium telluride, zinc telluride, thalliumbromoiodide, calcite, silicon carbide, cadmium lanthanum sulfide,strontium titanate, thallium chlorobromide, diamond, fabulite, zincselenide, cadmium selenide, gallium nitride, aluminum nitride, lithiumniobate, potassium niobate, germanium oxide, silver chloride, hafniumoxide, lithium iodate, zircon, yttrium oxide, silicon monoxide,sapphire, cesium iodide, lanthanum flint glass, cesium bromide, sodiumiodide, magnesium iodide, dense flint glass, lanthanum trifluoride,silica, lithium fluoride, magnesium fluoride, potassium fluoride, sodiumfluoride, and mixtures thereof; greater than ninety-five percent of saidUV-radiation scattering particles has particle diameters less than abouthalf of a maximum wavelength of UV radiation traversing saidsubstantially transparent material and said UV-radiation scatteringparticles have a mean particle diameter less than about one-tenth ofsaid wavelength; said first refractive index of said UV reflector isless than about 1.5 and said second refractive index of said UVreflector is greater than about 1.7; said plurality of layers comprisingan odd number of layers, each of said layers having a physical thicknessin a range from about 540 k/(4 n) to about 580 k/(4 n), wherein k is apositive integer selected from the group consisting of 1 and positiveeven integer numbers and where n is a refractive index of a material ofsaid layer measured at a light wavelength of 555 nm; a first layer ofsaid plurality of layers disposed adjacent to said shaped structure hassaid first refractive index; and said plurality of protrusions have ashape selected from the group consisting of conical, pyramidal, andhemispherical shapes, and said protrusions have a largest dimension in arange from about 540 k/(4 n) to about 580 k/(4 n), wherein k is apositive integer number selected from the group consisting of 1 andpositive even integer numbers and wherein n is a refractive index of amaterial of said protrusions, said largest dimensions being selectedfrom the group consisting of a height and a largest cross-sectionaldimension of a base of said protrusions.